1. Field of the Invention
The present invention relates to a magnetic disk unit, and particularly to a magnetic disk unit having a large recording density and excellent durability.
2. Description of the Prior Art
The magnetic disk unit usually comprises a hard disk unit or a floppy disk unit.
FIG. 1 shows an example of conventional hard disk unit.
The hard disk unit comprises a spindle 1, a disk-like magnetic recording medium (a magnetic disk) 2 driven by the spindle 1, and a magnetic head 3 whose sliding surface is spaced apart from the surface of the magnetic disk 2 by a predetermined distance. The magnetic head 3 is supported by a spring-like gimbal 4, which is supported by a suspension 5. The suspension 5 is cantilevered by a mover 6, which is movable along a radius of the magnetic disk 2. The magnetic disk 2 is made of an aluminum substrate.
When the magnetic disk 2 is rotated, an air flow, i.e., a thin air film is formed between the magnetic disk 2 and the magnetic head 3. This air film separates the magnetic head 3 from the magnetic disk 2 to cause no friction between them.
Unlike the hard disk unit, a conventional floppy disk unit allows, to some extent, friction to occur between a magnetic recording medium and a magnetic head. During use, a sliding surface of the magnetic head is in contact with the magnetic recording medium, which is made of a plastic substrate to reduce friction.
FIG. 2 is a diagram showing the relation of a relative velocity V to a frictional coefficient .mu. between a magnetic head and a magnetic recording medium of a magnetic disk unit. The diagram is called a Stribeck diagram. The frictional coefficient .mu. is expressed as follows: EQU .mu.=F/W
where F is a frictional force, and W is a load pushing the magnetic head downward.
As is apparent on the diagram, the frictional coefficient .mu. gradually decreases as the relative velocity V gradually decreases from its high-velocity state, and then the frictional coefficient suddenly increases after passing a minimum frictional coefficient point A. This phenomenon can be explained from a gap formed between the magnetic head and the magnetic recording medium. The gap is formed by the effect of an air film between the magnetic head and the magnetic recording medium in a range from the minimum frictional coefficient point A toward the higher velocity side on the Stribeck diagram. When the relative velocity V is reduced from the minimum frictional coefficient point A, the effect of the air film is reduced to reduce the gap and the magnetic head come in contact with the magnetic recording medium, thereby rapidly increasing the frictional coefficient .mu..
To avoid abrasion of the magnetic recording medium and magnetic head, the conventional hard disk unit is generally operated in a range AL of FIG. 2 where a sufficient gap is formed between the magnetic head and the magnetic recording medium. The range AL is defined by relative speeds VL1 and VL2.
Meanwhile, the magnetic recording medium suffers from an output loss, i.e., a spacing loss L which is expressed as follows: EQU L=K.multidot.(d/.lambda.)
where d is a distance from the magnetic head to the magnetic recording medium; .lambda. is a recording wavelength; and K is a proportional constant. To improve a recording density of the magnetic recording medium, the distance d must be as small as possible but is presently limited to about 0.2 .mu.m by various factors such as surface roughness of the magnetic recording medium.
In terms of the distance d and recording density that depend on the relative velocity V, the relative velocity VL1 of FIG. 2 of the conventional hard disk unit is quite large compared to a relative velocity at the minimum frictional coefficient point A.
As explained before, the conventional floppy disk unit allows friction to occur on the magnetic recording medium (a floppy disk) to some extent, because the floppy disk is replaceable. The floppy disk unit is operated, therefore, in a range AS of FIG. 2 with a slower relative velocity V to improve a recording density of the floppy disk. The range AS is defined by relative velocities VS1 and VS2 where a gap between a magnetic head and the magnetic recording medium is small while a frictional coefficient .mu. is very large. The maxium velocity VS2 of the range AS is very small compared to that at the minimum frictional coefficient point A, thereby maintaining a narrow gap between the magnetic head and the magnetic recording medium.
The conventional hard disk unit operated in the high velocity range AL and the conventional floppy disk unit operated in the low velocity range AS both raise problems, as explained above, in their recording densities and durability.